1. Field of the Invention
The present invention relates generally to a plasma processing system which is designed to produce plasma in a processing vessel by supplying radio-frequency waves to electrodes. More specifically, the invention relates to a plasma processing system wherein the waveform of radio-frequency waves supplied to electrodes is improved.
2. Description of the Related Background Art
FIG. 5A shows an example of a conventional plasma processing system. The plasma processing system shown in FIG. 5A has a processing vessel 1 for housing therein an object W to be processed (e.g., a semiconductor wafer or a flat panel display substrate). In the upper portion of the processing vessel 1, an electrode plate (top electrode) 2 having a facing surface 20 facing the object W is provided.
The plasma processing system also has a radio-frequency power supply 30 for supplying radio-frequency waves to the electrode plate 2. At the center of the top of the processing vessel 1, a feeding channel 4 having a coaxial structure for feeding radio-frequency waves to the electrode plate 2 from the radio-frequency power supply 30 is provided.
The plasma processing system also has a substantially cylindrical supporting table (bottom electrode) 12 provided on the bottom of the processing vessel 1. To the supporting table 12, an RF bias power supply 15 for applying a predetermined RF bias power is connected. In the bottom of the processing vessel 1 corresponding to the surrounding of the supporting table 12, an exhaust port 13 for evacuating the atmosphere of the processing vessel 1 is formed. At an appropriate position in the upper portion of the processing vessel 1, an introducing pipe 14 for introducing a process gas or the like is provided.
The plasma processing system is designed to produce the plasma of the process gas by radio-frequency waves, which are applied to the electrode plate 2, in the processing vessel 1 which is held at a predetermined degree of vacuum. Then, the plasma processing system is designed to carry out various plasma processes, such as deposition and etching processes, with respect to the object W on the supporting table 12.
In the above described plasma processing system, standing waves are formed by radio-frequency waves which are propagated in diametrically opposite directions on the facing surface 20 of the electrode plate 2. The radio-frequency waves supplied from the radio-frequency power supply 30 generally have a sinusoidal waveform.
Therefore, the standing waves formed by the radio-frequency waves also have a substantially sinusoidal waveform (except for that when amplitude is zero). For that reason, in accordance with the waveform of the standing waves, a field intensity distribution in radial directions of the object W is caused on the supporting table 12. For example, as shown in FIG. 5B, this field intensity distribution has a shape wherein a field intensity E gradually decreases from the central portion toward the outer peripheral portion in the direction of a radius r of the supporting table 12. For that reason, there is a problem in that the uniformity in the plasma process for the object W in the processing vessel decreases.
It is therefore an object of the present invention to eliminate the aforementioned problems and to provide a plasma processing system capable of decreasing the variation in field intensity in radial directions of an object to be processed, to more uniformly carry out a plasma process in a processing vessel.
In order to accomplish the aforementioned and other objects, according to the present invention, there is provided a plasma processing system comprising: a processing vessel for housing therein an object to be processed; an electrode provided in the processing vessel; and a radio-frequency wave supply unit for supplying a radio-frequency wave to the electrode, wherein a plasma is produced with the radio-frequency wave in the processing vessel, and the radio-frequency wave supply unit supplies radio-frequency wave having a flattened waveform which forms substantially a sinusoidal wave whose crest and trough portions are substantially horizontally flattened.
According to this plasma processing system, the standing waves formed on the electrode also have a flattened waveform which forms substantially a sinusoidal wave whose crest and trough portions are substantially horizontally flattened (except for that when amplitude is zero). Therefore, the variation in field intensity in radial directions of the object to be processed can be smaller than that when radio-frequency waves having a substantially sinusoidal waveform is used as it is. Therefore, it is possible to more uniformly plasma-process the object to be processed in the processing vessel.
In this plasma processing system, the radio-frequency wave supply unit may include: a radio-frequency wave source for producing a principal radio-frequency wave having a substantially sinusoidal waveform; a waveform shaper for waveform-shaping a part of the principal radio-frequency wave to produce a complementary radio-frequency wave having a waveform which forms only the crest and trough portions of the waveform of the principal radio-frequency wave; a phase adjuster for shifting the phase of the complementary radio-frequency wave, which has been produced by the waveform shaper, by 180 degrees with respect to the phase of the principal radio-frequency wave; and an adder for superposing the complementary radio-frequency wave, which has been phase-shifted by the phase adjuster, on the principal radio-frequency wave.
In this radio-frequency wave supply unit, by the superposition of the complementary radio-frequency wave on the principal radio-frequency wave in the adder, the crest and trough portions of the waveform of the principal radio-frequency wave can be canceled to obtain the radio-frequency wave having the flattened waveform which forms substantially a sinusoidal wave whose crest and trough portions are substantially horizontally flattened.
In the above described plasma processing system, the electrode may comprise a top electrode and a bottom electrode facing the top electrode, the radio-frequency wave having the flattened waveform being supplied to at least one of the top and bottom electrodes.